Glass for chemical strengthening

ABSTRACT

The present invention relates to a glass for chemical strengthening including, in mole percentage on an oxide basis: 60 to 72% of SiO 2 ; 9 to 20% of Al 2 O 3 ; 1 to 15% of Li 2 O; 0.1 to 5% of Y 2 O 3 ; 0 to 1.5% of ZrO 2 ; and 0 to 1% of TiO 2 , having a total content of one or more kinds of MgO, CaO, SrO, BaO and ZnO of 1 to 10%, having a total content of Na 2 O and K 2 O of 1.5 to 10%, having a total content of B 2 O 3  and P 2 O 5  of 0 to 10%, wherein a ratio ([Al 2 O 3 ]+[Li 2 O])/([Na 2 O]+[K 2 O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]+[ZrO 2 ]+[Y 2 O 3 ]) is from 0.7 to 3, wherein a ratio [MgO])/([CaO]+[SrO]+[BaO]+[ZnO]) is from 10 to 45, and having a value M expressed by the following expression of 1,100 or more: 
       M=−5×[SiO 2 ]+121×[Al 2 O 3 ]+50×[Li 2 O]−35×[Na 2 O]+32×[K 2 O]+85×[MgO]+54×[CaO]−41×[SrO]−4×[P 2 O 5 ]+218×[Y 2 O 3 ]+436×[ZrO 2 ]−1180,
         wherein each of [SiO 2 ], [Al 2 O 3 ], [Li 2 O], [Na 2 O], [K 2 O], [MgO], [CaO], [SrO], [P 2 O 5 ], [Y 2 O 3 ], and [ZrO 2 ] designates a content of each component in mole percentage on an oxide basis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of prior U.S. applicationSer. No. 16/911,878, filed Jun. 25, 2020, the disclosure of which isincorporated herein by reference in its entirety. U.S. application Ser.No. 16/911,878 is a Continuation of PCT/JP2018/037115, filed Oct. 3,2018, the disclosure of which is incorporated herein by reference in itsentirety. U.S. application Ser. No. 16/911,878 claims priority toJapanese Application No. 2018-018508, filed Feb. 5, 2018, the disclosureof which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a glass for chemical strengthening.

BACKGROUND ART

In recent years, a cover glass made of a chemically strengthened glassis used in order to protect a display device of a mobile apparatus suchas a cellular phone, a smartphone, a personal digital assistant (PDA),or a tablet terminal and to enhance the appearance thereof.

The chemically strengthened glass has a tendency that the strengththereof become higher as a surface compressive stress (value) (CS) or adepth of compressive stress layer (DOL) thereof increases. On the otherhand, an internal tensile stress (CT) occurs inside the glass so as tokeep balance with the surface compressive stress. Therefore, the CTbecomes larger as the CS or DOL increases. When a glass having a largeCT is broken, the cracking manner is vigorous with a large number ofbroken pieces, thereby increasing a risk that the broken pieces mayscatter.

Patent Literature 1 describes that a stress profile expressed by a bentline is formed by two stages of chemical strengthening treatment so thata surface compressive stress can be increased while suppressing aninternal tensile stress. According to a specific method proposedtherein, a KNO₃/NaNO₃ mixed salt comparatively low in K concentration isused in the first stage of the chemical strengthening treatment, and aKNO₃/NaNO₃ mixed salt comparatively high in K concentration is used inthe second stage of the chemical strengthening treatment.

In addition, Patent Literature 2 discloses a lithium aluminosilicateglass capable of obtaining a comparatively large surface compressivestress and a comparatively large depth of compressive stress layer bytwo stages of chemical strengthening treatment. In the lithiumaluminosilicate glass, both the CS and the DOL can be increased by twostages of chemical strengthening treatment in which a sodium salt isused in the first stage of the chemical strengthening treatment, and apotassium salt is used in the second stage of the chemical strengtheningtreatment.

CITATION LIST Patent Literature

-   Patent Literature 1: US 2015/0259244-   Patent Literature 2: JP-T-2013-520388

SUMMARY OF INVENTION Technical Problem

Recently, demand for thinner and lighter cover glasses and cover glassesprocessed into curved shapes is increasing. Attention is paid to alithium aluminosilicate glass capable of simultaneously increasing asurface compressive stress value (CS) and a depth of compressive stresslayer (DOL).

However, the lithium aluminosilicate glass tends to be devitrified in astep of manufacturing the glass or in a step of bending or the like theobtained glass.

An object of the present invention is to provide a glass for chemicalstrengthening which is hardly devitrified but can achieve a large CS anda large DOL.

Solution to Problem

The present invention provides a glass for chemical strengtheningincluding, in mole percentage on an oxide basis:

45 to 75% of SiO₂;

1 to 30% of Al₂O₃;

1 to 20% of Li₂O;

0 to 5% of Y₂O₃;

0 to 5% of ZrO₂; and

0 to 1% of TiO₂,

having a total content of one or more kinds of MgO, CaO, SrO, BaO andZnO of 1 to 20%,

having a total content of Na₂O and K₂O of 0 to 10%,

having a total content of B₂O₃ and P₂O₅ of 0 to 10%, and

having a value M expressed by the following expression of 1,000 or more:

M=−5×[SiO₂]+121×[Al₂O₃]+50×[Li₂O]−35×[Na₂O]+32×[K₂O]+85×[MgO]+54×[CaO]−41×[SrO]−4×[P₂O₅]+218×[Y₂O₃]+436×[ZrO₂]−1180.

In addition, the present invention provides a glass for chemicalstrengthening in which when the glass is chemically strengthened by animmersion in a sodium nitrate at 450° C. for 1 hour, a surfacecompressive stress value thereof is 300 MPa or more, and

when the glass is chemically strengthened by an immersion in a sodiumnitrate at 450° C. for 3 hour and a subsequent immersion in a potassiumnitrate at 450° C. for 1.5 hours, a surface compressive stress valuethereof is 800 MPa or more, and

a chemically strengthened glass has a base composition including, inmole percentage on an oxide basis:

45 to 75% of SiO₂;

1 to 30% of Al₂O₃;

1 to 20% of Li₂O;

0 to 5% of Y₂O₃;

0 to 5% of ZrO₂; and

0 to 1% of TiO₂,

having a total content of one or more kinds of MgO, CaO, SrO, BaO andZnO of 1 to 20%,

having a total content of Na₂O and K₂O of 0 to 10%,

having a total content of B₂O₃ and P₂O₅ of 0 to 10%, and

having a value M expressed by the above-described expression of 1,000 ormore.

Advantageous Effects of Invention

According to the present invention, it is possible to provide achemically strengthened glass which is hardly devitrified but has alarge surface compressive stress value (CS) and a large depth ofcompressive stress layer (DOL).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a relationship between a value I and a crystalgrowth rate.

FIG. 2 is a view showing a relationship between a value I2 and adevitrification property.

FIG. 3 is an example of a graph showing a relationship between a fictivetemperature and a refractive index.

FIG. 4 is a schematic view of a platinum vessel used for measuring acrystal growth rate.

FIG. 5 is a temperature profile on float simulation temperature droppingconditions.

DESCRIPTION OF EMBODIMENTS

A glass for chemical strengthening of the present invention will bedescribed in detail below. However, the present invention is not limitedto the following embodiments and can be carried out in any modifiedmanner as long as not departing from the gist of the present invention.

In the present description, the phrase “chemically strengthened glass”designates a glass which has been subjected to a chemical strengtheningtreatment, and the phrase “glass for chemical strengthening” designatesa glass which has not been subjected to a chemical strengtheningtreatment yet.

In the present description, a glass composition of a glass for chemicalstrengthening may be referred to as a base composition of a chemicallystrengthened glass. Normally in a chemically strengthened glass, acompressive stress layer caused by ion exchange is formed in a surfacepart of the glass. Therefore, a part excused from the ion exchange has aglass composition coinciding with a base composition of the chemicallystrengthened glass.

In the present description, any glass composition is represented by molepercentage on an oxide basis, and mol % may be abbreviated to %. Inaddition, the word “to” designating a numerical range is used as adenotation of a range including numerical values before and after theword “to” as a lower limit value and an upper limit value of the range.

In a glass composition, the phrase “substantially not contained” meansnot to be contained but unavoidable impurities contained in rawmaterials etc., that is, means not to be intentionally added.Specifically, the phrase means that the content in the glass compositionis, for example, lower than 0.1 mol %.

The phrase “stress profile” in the present description designates apattern expressing a compressive stress value with a depth from a glasssurface as a variable. A negative compressive stress value means atensile stress.

The word “frangibility” of a glass in the present description means aproperty that broken pieces are easy to scatter when the glass isbroken.

In the following description, unless otherwise stated, the “surfacecompressive stress value” of a glass is a surface compressive stressvalue obtained by performing a chemical strengthening treatment on aglass sheet obtained by retaining a glass sheet having a thickness of0.8 mm for one or more hours at a temperature 50° C. higher than a glasstransition point Tg of the glass and then gradually cooling down theglass sheet at a cooling rate of 0.5° C./min. The surface compressivestress generated by the chemical strengthening treatment tends toincrease as the fictive temperature of the glass is lower. The fictivetemperature of the glass is influenced by a thermal history (forexample, the cooling rate) which the glass received. The influence ofthe fictive temperature is eliminated to evaluate the surfacecompressive stress by the heat treatment and the gradual cooling.

In addition, a “β-OH value” is obtained by Expression (1) from atransmittance X₁ (%) measured at a reference wavelength of 4,000 cm⁻¹ byan FT-IR method, a minimum transmittance X₂ (%) near 3,570 cm⁻¹ which isan absorption wavelength of hydroxyl groups, and a thickness t (unit:mm) of the glass sheet.

β-OH value=(1/t)log₁₀(X₁/X₂)  (1)

Incidentally, the β-OH value can be adjusted by the amount of moisturecontained in glass raw materials or the melting conditions.

<Glass for Chemical Strengthening>

A glass for chemical strengthening of the present invention (hereinafteroften referred to as “glass of the present invention”) includes, in molepercentage on an oxide basis:

45 to 75% of SiO₂;

1 to 30% of Al₂O₃;

1 to 20% of Li₂O;

0 to 5% of Y₂O₃;

0 to 5% of ZrO₂; and

0 to 1% of TiO₂, in which,

a total content of one or more kinds of MgO, CaO, SrO, BaO and ZnO is 1to 20%,

a total content of Na₂O and K₂O is 0 to 10%, and

a total content of B₂O₃ and P₂O₅ is 0 to 10%.

The glass of the present invention does not only satisfy theaforementioned composition ranges, but has a value M expressed by thefollowing expression of 1,000 or more,

M=−5×[SiO₂]+121×[Al₂O₃]+50×[Li₂O]−35×[Na₂O]+32×[K₂O]+85×[MgO]+54×[CaO]-41×[SrO]-4×[P₂O₅]+218×[Y₂O₃]+436×[ZrO₂]−1180.

In the expression, [SiO₂], [Al₂O₃], [Li₂O], [Na₂O], [K₂O], [MgO], [CaO],[SrO], [P₂O₅], [Y₂O₃], and [ZrO₂] designate contents of respectivecomponents represented by mole percentage. The same thing can be appliedto the following description.

The present inventors studied the relationship between a glasscomposition of a glass for chemical strengthening and a compressivestress value caused by a chemical strengthening and the relationshipbetween a glass composition of a glass for chemical strengthening andchemical strengthening property, and found that, for a glass large inthe aforementioned value M, a large compressive stress value could beintroduced by a chemical strengthening treatment, and devitrification(crystal) hardly occurred.

The present inventors investigated various glass compositions as to asurface compressive stress in each glass subjected to a chemicalstrengthening treatment, and a crystal growth rate at 850° C. to 1,200°C. As conditions for the chemical strengthening treatment, evaluationwas performed on a case (one-stage chemical strengthening treatment)where a glass was chemically strengthened by immersion in sodium nitrateat 450° C. for 1 hour, and a case (two-stage chemical strengtheningtreatment) where a glass was chemically strengthened by immersion insodium nitrate at 450° C. for 3 hours and subsequent immersion inpotassium nitrate at 450° C. for 1.5 hours.

As a result, in a glass having a value M of 1,000 or more, a surfacecompressive stress (CS1) after the one-stage chemical strengtheningtreatment in which the glass was immersed in sodium nitrate at 450° C.for 1 hour reached 300 MPa or more. On the other hand, a surfacecompressive stress (CS2) after the two-stage chemical strengtheningtreatment in which the glass was immersed in sodium nitrate at 450° C.for 3 hours and subsequently immersed in potassium nitrate at 450° C.for 1.5 hours reached 800 MPa or more.

In the glass large in the value M, the crystal growth rate at 850 to1,200° C. was low, and there was also confirmed a tendency that a defectcaused by devitrification during manufacturing of the glass wasinhibited. A method for evaluating the crystal growth rate will bedescribed later.

The value M is more preferably 1,100 or more, and even more preferably1,200 or more. However, when the value M is too large, there is aconcern that the glass may be brittle. Therefore, the value M ispreferably 1,800 or less, more preferably 1,650 or less, even morepreferably 1,500 or less, and typically 1,350 or less.

In addition, the glass of the present invention does not only satisfythe aforementioned composition ranges, but has a value I expressed bythe following expression of preferably 850 or less, and more preferably600 or less.

I=−4.8×[SiO₂]+102×[Al₂O₃]+81×[Li₂O]−272×[Na₂O]−281×[K₂O]−16×[MgO]−25×[Y₂O₃]+0.028×[ZrO₂]+63

The present inventors studied a glass composition and a devitrificationproperty of a glass for chemical strengthening, and found a highcorrelation between the aforementioned value I and a crystal growth rateat 700 to 1,200° C. in a glass which will be described later.

FIG. 1 is a graph in which the relationship between the value I and thecrystal growth rate is plotted as to Examples and Comparative Exampleswhich will be described later.

When the value I is preferably 850 or less and more preferably 600 orless, the glass is hardly devitrified when the glass is manufactured.The value I is more preferably 500 or less, even more preferably 400 orless, particularly preferably 300 or less, and most preferably 200 orless. The lower limit of the value I is not particularly limited.However, the value I is preferably over 50, more preferably 80 or more,and particularly preferably 100 or more, in consideration of acompressive stress value caused by chemical strengthening of the glass.

In addition, the present inventors found that the glass of the presentinvention does not only satisfy the aforementioned composition ranges,but has a value I2 expressed by the following expression of preferably 5or less.

I2=0.27×[SiO₂]+1.4×[Al₂O₃]−1.1×[Na₂O]−1.7×[K₂O]+0.38×[MgO]−1.36×[Y₂O₃]−0.59×[ZrO₂]−23

The present inventors studied a glass composition and a devitrificationproperty of a glass for chemical strengthening, and found that when aglass having a glass composition small in the aforementioned value I2was hardly devitrified when the glass was cooled down from a temperaturenot lower than a liquidus temperature of the glass.

The present inventors prepared glasses (A1 to A18) having compositionsshown in Table 1 and Table 2. Two-stage heat treatment for retainingeach glass at 1,300° C. for 30 minutes and then retaining the glass at atemperature shown in each second-stage treatment temperature field (850°C. to 1,200° C.) for 10 minutes was performed, and presence or absenceof devitrification was observed. In Tables 1 and 2, presence or absenceof devitrification is written for each second-stage treatmenttemperature, and the number of temperature conditions wheredevitrification occurred is added. In the tables, “N” designates that nodevitrification was observed, and “●” designates that devitrificationwas observed. For example, in the case of the glass A3, devitrificationwas observed at two conditions of 850° C. and 900° C. Therefore, thenumber of temperature conditions where devitrification occurred is 2. Itcan be said that a glass small in “the number of temperature conditionswith devitrification in two-stage heat treatment” meaning “the number oftemperature conditions where devitrification occurred” has a lowpossibility to be devitrified when cooled down from high temperature,and is excellent in resistance to devitrification.

TABLE 1 A1 A2 A3 A4 A5 A6 A7 A8 A9 SiO₂ 70.00  69.62  69.43  69.24 68.86  68.30  67.73 67.16 66.60 Al₂O₃ 7.50 7.99 8.24 8.49 8.98 9.7210.46 11.19 11.93 MgO 7.00 6.67 6.49 6.32 5.99 5.48 4.98 4.48 3.98 CaO0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 ZrO₂ 1.00 1.00 1.00 1.001.00 1.00 1.00 1.00 0.99 Y₂O₃ — — — — — — — — — Li₂O 8.00 8.49 8.74 8.999.48 10.22  10.96 11.69 12.43 Na₂O 5.30 5.03 4.90 4.76 4.49 4.09 3.693.28 2.88 K₂O 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.99 Value I2 0.941.69 2.08 2.47 3.22 4.35 5.49 6.62 7.75  850° C. N N • • • • • • •  900°C. N • • • • • • • •  950° C. N N N • • • • • • 1000° C. N N N • • • • •• 1050° C. N N N N N • • • • 1100° C. N N N N N N • • • 1150° C. N N N NN N N N • 1200° C. N N N N N N N N N Number of 0   1   2   4   4   5   66 7 temperature conditions with devitrification in two-stage thermaltreatment Devitrification on N N N N N N • • • float simulationtemperature dropping condition

TABLE 2 A10 A11 A12 A13 A14 A15 A16 A17 A18 SiO₂ 71.00  63.20  65.0065.00 63.00 63.30 63.00 69.00 67.00 Al₂O₃ 7.00 14.30  11.00 11.00 12.0011.00 10.00 11.00 11.50 MgO 6.00 1.00 4.90 4.90 5.90 5.90 6.90 1.90 2.90CaO — — 0.20 0.20 0.20 0.20 0.20 0.20 0.20 ZrO₂ — 1.00 1.00 1.00 1.001.00 1.00 1.00 1.00 Y₂O₃ — — — — — — 1.00 — — Li₂O 8.50 13.50  11.0011.00 11.00 11.00 11.00 10.00 10.50 Na₂O 7.50 2.00 5.80 4.80 2.80 4.305.80 5.80 4.80 K₂O — 5.00 1.10 2.10 4.10 3.30 1.10 1.10 2.10 Value I20.00 3.17 2.97 2.37 2.41 0.80 0.43 2.91 2.85  850° C. N • • • • N N N • 900° C. N • • • • N N • N  950° C. N • • • N N N • N 1000° C. N • • N NN N N N 1050° C. N N N N N N N N N 1100° C. N N N N N N N N N 1150° C. NN N N N N N N N 1200° C. N N N N N N N N N Number of 0   4   4 3 2 0 0 21 temperature conditions with devitrification in two-stage thermaltreatment Devitrification on N N N N N N N N N float simulationtemperature dropping condition

The value I2 is preferably 5 or less, since the glass is hardlydevitrified when it is melted. The value I2 is more preferably 4 orless, even more preferably 3 or less, particularly preferably 2 or less,and most preferably 1 or less. The lower limit of the value I2 is notparticularly limited. In consideration of the brittleness of the glass,the value I2 is preferably over −5, more preferably −3 or more, andparticularly preferably −1 or more.

Tables 1 and 2 also show results of observation as to presence orabsence of devitrification at high temperature when each glass wascooled down on conditions simulating cooling conditions of a case wherea glass sheet was manufactured by a float process (hereinafter referredto as “float simulation temperature dropping conditions”). Therelationship between the results of those devitrification tests and thevalues I2 is plotted in FIG. 2 . From FIG. 2 , it is proved that a glasshaving a small value I2 is hardly devitrified when it is shaped into aglass sheet by a float process. Particularly, a glass having a value I2of 5 or less was hardly devitrified.

On the float simulation temperature dropping conditions, the averagecooling rate from 1,200° C. to 600° C. is about 15° C./min. A glasshaving a large number of temperature conditions where devitrificationoccurred in the aforementioned two-stage treatment test was alsodevitrified in this test.

Incidentally, in the glass of the present invention satisfying theaforementioned composition ranges, it is preferable that at least onekind of the value M, the value I and the value I2 satisfies theaforementioned ranges. It is also preferable that two kinds or all thethree kinds of them satisfy the aforementioned ranges.

A preferred glass composition will be described below.

SiO₂ is a component forming a network of the glass. In addition, SiO₂ isa component enhancing the chemical durability, and it is also acomponent reducing occurrence of cracking when the surface of the glassis damaged.

The content of SiO₂ is preferably 45% or higher. The content of SiO₂ ismore preferably 55% or higher, even more preferably 60% or higher,particularly preferably 63% or higher, and typically 65% or higher. Onthe other hand, in order to enhance the meltability, the content of SiO₂is 75% or lower, preferably 72% or lower, more preferably 70% or lower,and particularly preferably 68% or lower.

Al₂O₃ is an effective component in terms of improvement of ion exchangeperformance in chemical strengthening and increase in a surfacecompressive stress after the chemical strengthening.

The content of Al₂O₃ is preferably 1% or higher. Al₂O₃ is a componentraising the glass transition point and also a component increasing theYoung's modulus. The content of Al₂O₃ is preferably 8% or higher, morepreferably 9% or higher, even more preferably 10% or higher,particularly preferably 11% or higher, and typically 12% or higher. Onthe other hand, when the content of Al₂O₃ is too high, the crystalgrowth rate increases to aggravate a problem of reduction in yieldcaused by a defect of devitrification. In addition, the viscosity of theglass increases to lower the meltability. The content of Al₂O₃ is 30% orlower, preferably 20% or lower, more preferably 18% or lower, even morepreferably 16% or lower, and typically 14% or lower.

Li₂O is a component forming a surface compressive stress by the effectof ion exchange, and a component improving the meltability of the glass.When the chemically strengthened glass contains Li₂O, a stress profilelarge in both the surface compressive stress and the compressive stresslayer can be obtained by a method in which Li ions in the glass surfaceare ion-exchanged by Na ions, and Na ions are ion-exchanged by K ions.In order to easily obtain a preferred stress profile, the content ofLi₂O is preferably 1% or higher, more preferably 5% or higher, even morepreferably 7% or higher, particularly preferably 9% or higher, andtypically 10% or higher.

On the other hand, when the content of Li₂O is too high, the crystalgrowth rate during melting of the glass increases to aggravate a problemof reduction in yield caused by a defect of devitrification. The contentof Li₂O is preferably 20% or lower, more preferably 15% or lower, evenmore preferably 13% or lower, and typically 12% or lower.

Na₂O and K₂O are not essential, but they are components improving themeltability of the glass and decreasing the crystal growth rate of theglass. Na₂O and K₂O may be added in order to improve the ion exchangeperformance.

Na₂O is a component forming a surface compressive stress layer in achemical strengthening treatment using a potassium salt, and also acomponent which can improve the meltability of the glass.

In order to obtain those effects, the content of Na₂O is preferably 1%or higher, more preferably 2% or higher, even more preferably 3% orhigher, particularly preferably 4% or higher, and typically 5% orhigher. On the other hand, in order to avoid reduction in surfacecompressive stress (CS) in a strengthening treatment using a sodiumsalt, the content of Na₂O is preferably 10% or lower, more preferably 8%or lower, even more preferably 6% or lower, and particularly preferably5% or lower.

K₂O may be contained for a purpose such as improving the ion exchangeperformance. When K₂O is contained, the content of K₂O is preferably0.5% or higher, more preferably 1% or higher, even more preferably 1.5%or higher, particularly preferably 2% or higher, and typically 3% orhigher. On the other hand, in order to avoid reduction in surfacecompressive stress (CS) caused by a potassium salt, the content of K₂Ois preferably 10% or lower, more preferably 5% or lower, even morepreferably 3% or lower, and particularly preferably 2% or lower.

The total content ([Na₂O]+[K₂O]) of Na₂O and K₂O is preferably 0 to 10%,more preferably 5% or higher, and even more preferably 6% or higher. Onthe other hand, the total content is more preferably 8% or lower, andparticularly preferably 7% or lower.

In order to reduce the growth rate of devitrification,[Li₂O]/([Na₂O]+[K₂O]) is preferably 3 or lower, more preferably 2.5 orlower, and even more preferably 2 or lower. On the other hand, in orderto increase the surface compressive stress in the chemical strengtheningtreatment using sodium, [Li₂O]/([Na₂O]+[K₂O]) is preferably 0.5 orhigher, more preferably 0.9 or higher, and even more preferably 1.3 orhigher.

In addition, in order to reduce the growth rate of devitrification,([Al₂O₃]+[Li₂O])/([Na₂O]+[K₂O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]+[ZrO₂]+[Y₂O₃])is preferably 4 or lower, more preferably 3 or lower, and even morepreferably 2 or lower. On the other hand, in order to increase thesurface compressive stress in the chemical strengthening treatment usingsodium,([Al₂O₃]+[Li₂O])/([Na₂O]+[K₂O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]+[ZrO₂]+[Y₂O₃])is preferably 0.5 or higher, more preferably 0.7 or higher, even morepreferably 0.9 or higher, and particularly preferably 1 or higher.

MgO, CaO, SrO, BaO and ZnO are not essential. However, in order toenhance the stability of the glass, it is preferable to contain one ormore kinds of them. The total content [MgO]+[CaO]+[SrO]+[BaO]+[ZnO] ofthem is preferably 1% or higher, more preferably 2% or higher, even morepreferably 3% or higher, and particularly preferably 4% or higher. Onthe other hand, in order to improve the ion exchange performance in thechemical strengthening, the total content is preferably 20% or lower,more preferably 15% or lower, even more preferably 10% or lower, andfurther more preferably 8% or lower.

When one or more kinds of MgO, CaO, SrO, BaO and ZnO are contained,[MgO]/([CaO]+[SrO]+[BaO]+[ZnO]) is preferably 10 or higher, morepreferably 15 or higher, even more preferably 20 or higher, andparticularly preferably 25 or higher, in order to lower the surfacereflectivity of the glass. Since CaO, SrO, BaO and ZnO increase thereflectivity in comparison with MgO. In order to lower thedevitrification temperature, [MgO]/([CaO]+[SrO]+[BaO]+[ZnO]) ispreferably 60 or lower, more preferably 55 or lower, even morepreferably 50 or lower, and particularly preferably 45 or lower.

MgO is preferably contained in order to reduce the crystal growth ratewhile increasing the meltability of the chemically strengthened glass.The content of MgO is preferably 1% or higher, more preferably 2% orhigher, even more preferably 3% or higher, particularly preferably 4% orhigher, and typically 5% or higher. On the other hand, when the contentof MgO is too high, the compressive stress layer is hardly increased inthe chemical strengthening treatment. The content of MgO is preferably15% or lower, more preferably 10% or lower, even more preferably 8% orlower, and particularly preferably 6% or lower.

CaO is a component improving the meltability of the glass for chemicalstrengthening. CaO may be contained. When CaO is contained, the contentof CaO is preferably 0.1% or higher, more preferably 0.15% or higher,and even more preferably 0.5% or higher. On the other hand, when thecontent of CaO is excessive, the compressive stress value is hardlyincreased in the chemical strengthening treatment. The content of CaO ispreferably 5% or lower, more preferably 3% or lower, even morepreferably 1% or lower, and typically 0.5% or lower.

SrO is a component improving the meltability of the glass for chemicalstrengthening. SrO may be contained. When SrO is contained, the contentof SrO is preferably 0.1% or higher, more preferably 0.15% or higher,and even more preferably 0.5% or higher. On the other hand, when thecontent of SrO is excessive, the compressive stress value is hardlyincreased in the chemical strengthening treatment. The content of SrO ispreferably 3% or lower, more preferably 2% or lower, even morepreferably 1% or lower, and typically 0.5% or lower.

BaO is a component improving the meltability of the glass for chemicalstrengthening. BaO may be contained. When BaO is contained, the contentof BaO is preferably 0.1% or higher, more preferably 0.15% or higher,and even more preferably 0.5% or higher. On the other hand, when thecontent of BaO is excessive, the compressive stress value is hardlyincreased in the chemical strengthening treatment. The content of BaO ispreferably 3% or lower, more preferably 2% or lower, even morepreferably 1% or lower, and typically 0.5% or lower.

ZnO is a component improving the meltability of the glass for chemicalstrengthening. ZnO may be contained. When ZnO is contained, the contentof ZnO is preferably 0.1% or higher, more preferably 0.15% or higher,and even more preferably 0.5% or higher. On the other hand, when thecontent of ZnO is excessive, the compressive stress value is hardlyincreased in the chemical strengthening treatment. The content of ZnO ispreferably 3% or lower, more preferably 2% or lower, even morepreferably 1% or lower, and typically 0.5% or lower.

ZrO₂ does not have to be contained. However, it is preferable to containZrO₂ in order to increase the surface compressive stress of thechemically strengthened glass. The content of ZrO₂ is preferably 0.1% orhigher, more preferably 0.2% or higher, even more preferably 0.5% orhigher, particularly preferably 0.8% or higher, and typically 1% orhigher. On the other hand, when the content of ZrO₂ is too high, thecompressive stress value is hardly increased in the chemicalstrengthening treatment. The content of ZrO₂ is preferably 5% or lower,more preferably 3% or lower, even more preferably 2% or lower, andparticularly preferably 1.5% or lower.

TiO₂ is a component inhibiting solarization of the glass. TiO₂ may becontained. When TiO₂ is contained, the content of TiO₂ is preferably0.02% or higher, more preferably 0.05% or higher, even more preferably0.1% or higher, particularly preferably 0.12% or higher, and typically0.15% or higher. On the other hand, when the content of TiO₂ is beyond1%, there is a concern that devitrification may occur easily todeteriorate the quality of the chemically strengthened glass. Thecontent of TiO₂ is preferably 1% or lower, more preferably 0.5% orlower, and even more preferably 0.25% or lower.

Y₂O₃ does not have to be contained. However, Y₂O₃ is a componentreducing the crystal growth rate while increasing the surfacecompressive stress of the chemically strengthened glass. Therefore, Y₂O₃is preferably contained. The content of Y₂O₃ is preferably 0.1% orhigher, more preferably 0.2% or higher, even more preferably 0.5% orhigher, particularly preferably 0.8% or higher, and typically 1% orhigher. On the other hand, when the content of Y₂O₃ is too high, thecompressive stress layer is hardly increased in the chemicalstrengthening treatment. The content of Y₂O₃ is preferably 5% or lower,more preferably 3% or lower, even more preferably 2% or lower, andparticularly preferably 1.5% or lower.

B₂O₃ is not essential. However, B₂O₃ may be contained in order to reducethe brittleness of the glass to improve the crack resistance, and toimprove the meltability of the glass. When B₂O₃ is contained, thecontent of B₂O₃ is preferably 0.5% or higher, preferably 1% or higher,and even more preferably 2% or higher. On the other hand, when thecontent of B₂O₃ is too high, the acid resistance may deteriorate easily.Therefore, the content of B₂O₃ is preferably 10% or lower. The contentof B₂O₃ is more preferably 6% or lower, even more preferably 4% orlower, and typically 2% or lower. In order to prevent a problem thatstriae may occur during melting, it is more preferable that B₂O₃ issubstantially not contained.

P₂O₅ is not essential. However, P₂O₅ may be contained in order toincrease the compressive stress layer in chemical strengthening. WhenP₂O₅ is contained, the content of P₂O₅ is preferably 0.5% or higher,preferably 1% or higher, and even more preferably 2% or higher. On theother hand, in order to enhance the acid resistance, the content of P₂O₅is preferably 6% or lower, more preferably 4% or lower, and even morepreferably 2% or lower. In order to prevent a problem that striae mayoccur during melting, it is more preferable that P₂O₅ is substantiallynot contained.

The total content of B₂O₃ and P₂O₅ is preferably 0 to 10%. The lowerlimit of the total content is more preferably 1% or higher, and morepreferably 2% or higher. On the other hand, the total content of B₂O₃and P₂O₅ is preferably 6% or lower, and more preferably 4% or lower.

La₂O₃, Nb₂O₅, Ta₂O₅ and Gd₂O₃ are components reducing the crystal growthrate of the glass and improving the meltability. La₂O₃, Nb₂O₅, Ta₂O₅ andGd₂O₃ may be contained. When those components are contained, the contentof each component is preferably 0.1% or higher, more preferably 0.2% orhigher, even more preferably 0.5% or higher, particularly preferably0.8% or higher, and typically 1% or higher. On the other hand, when thecontent of those components is too high, the compressive stress value ishardly increased in the chemical strengthening treatment. Therefore, thecontent of each component is preferably 3% or lower, more preferably 2%or lower, even more preferably 1% or lower, and particularly preferably0.5% or lower.

Fe₂O₃ absorbs heat rays and thus has an effect of improving themeltability of the glass. For mass production of glasses using alarge-scale melting furnace, it is preferable to contain Fe₂O₃. In thatcase, the content of Fe₂O₃ is preferably 0.002% or higher, morepreferably 0.005% or higher, even more preferably 0.007% or higher, andparticularly preferably 0.01% or higher, as represented by weight % onthe oxide basis. On the other hand, when Fe₂O₃ is excessively contained,the glass is colored. In order to enhance the transparency of the glass,the content of Fe₂O₃ is preferably 0.3% or lower, more preferably 0.04%or lower, even more preferably 0.025% or lower, and particularlypreferably 0.015% or lower, as represented by weight % on the oxidebasis.

Incidentally, description has been made here on the assumption that allthe iron oxides in the glass are Fe₂O₃. In fact, however, it is commonthat Fe (III) in an oxidation state and Fe (II) in a reduction state aremixed. Of them, Fe (III) colors the glass in yellow, and Fe (II) colorsthe glass in blue. In accordance with the balance between the both, theglass is colored in green.

Further, coloring components may be added as long as not impeding theattainment of desired chemical strengthening property. Preferredexamples of the coloring components include Co₃O₄, MnO₂, NiO, CuO,Cr₂O₃, V₂O₅, Bi₂O₃, SeO₂, CeO₂, Er₂O₃, and Nd₂O₃.

The total content of the coloring components is preferably 5% or lower,as represented by mole percentage on the oxide basis. When the totalcontent exceeds 5%, the glass may be devitrified easily. The content ofthe coloring components is preferably 3% or lower, and more preferably1% or lower. In order to increase the transmittance of the glass, it ispreferable that those components are substantially not contained.

SO₃, chlorides, fluorides, etc. may be contained properly as refiningagents during melting of the glass. It is preferable that As₂O₃ is notcontained. When Sb₂O₃ is contained, the content of Sb₂O₃ is preferably0.3% or lower, more preferably 0.1% or lower, and most preferablysubstantially not contained.

The β-OH value of the glass of the present invention is preferably 0.1mm⁻¹ or more, more preferably 0.15 mm⁻¹ or more, even more preferably0.2 mm⁻¹ or more, particularly preferably 0.22 mm⁻¹ or more, and mostpreferably 0.25 mm⁻¹ or more.

A glass large in β-OH value which is an index of the amount of moisturein the glass is low in softening point so as to tend to be easily bent.On the other hand, from the viewpoint of improvement of strength bychemical strengthening of the glass, the large β-OH value of the glassdecreases the value of the surface compressive stress (CS) after thechemical strengthening treatment so that it is difficult to improve thestrength. Therefore, the β-OH value is preferably 0.5 mm⁻¹ or less, morepreferably 0.4 mm⁻¹ or less, and even more preferably 0.3 mm⁻¹ or less.

The Young's modulus of the glass of the present invention is preferably80 GPa or more, more preferably 82 GPa or more, even more preferably 84GPa or more, and particularly preferably 85 GPa or more, in order toimprove the frangibility of the glass. The upper limit of the Young'smodulus is not particularly limited. However, a glass high in Young'smodulus may be low in acid resistance. Therefore, the Young's modulusis, for example, 110 GPa or less, preferably 100 GPa or less, and morepreferably 90 GPa or less. The Young's modulus can be, for example,measured by an ultrasonic pulse method.

The density of the glass of the present invention is preferably 3.0g/cm³ or lower, more preferably 2.8 g/cm³ or lower, even more preferably2.6 g/cm³ or lower, and particularly preferably 2.55 g/cm³ or lower, inorder to reduce the weight of a product. The lower limit of the densityis not particularly limited. However, a glass having a low density tendsto have a low acid resistance or the like. The density is, for example,2.3 g/cm³ or higher, preferably 2.4 g/cm³ or higher, and particularlypreferably 2.45 g/cm³ or higher.

The refractive index of the glass of the present invention is preferably1.6 or lower, more preferably 1.58 or lower, even more preferably 1.56or lower, and particularly preferably 1.54 or lower, in order to reducesurface reflection of visible light. The lower limit of the refractiveindex is not particularly limited. However, a glass having a lowrefractive index tends to have a low acid resistance. Therefore therefractive index is, for example, 1.5 or higher, preferably 1.51 orhigher, and more preferably 1.52 or higher.

The photoelastic constant of the glass of the present invention ispreferably 33 nm/cm/MPa or lower, more preferably 32 nm/cm/MPa or lower,even more preferably 31 nm/cm/MPa or lower, and particularly preferably30 nm/cm/MPa or lower, in order to reduce an optical strain. On theother hand, a glass having a low photoelastic constant tends to have alow acid resistance. Therefore, the photoelastic constant of the glassof the present invention is, for example, 24 nm/cm/MPa or higher, morepreferably 25 nm/cm/MPa or higher, and even more preferably 26 nm/cm/MPaor higher.

The average linear thermal expansion coefficient (thermal expansioncoefficient) at 50 to 350° C. in the glass of the present invention ispreferably 95×10⁻⁷/° C. or lower, more preferably 90×10⁻⁷/° C. or lower,even more preferably 88×10⁻⁷/° C. or lower, particularly preferably86×10⁻⁷/° C. or lower, and most preferably 84×10⁻⁷/° C. or lower. Thelower limit of the thermal expansion coefficient is not particularlylimited. However, a glass having a low thermal expansion coefficient maybe hardly melted. Therefore, the average linear thermal expansioncoefficient (thermal expansion coefficient) at 50 to 350° C. in theglass of the present invention is, for example, 60×10⁻⁷/° C. or higher,preferably 70×10⁻⁷/° C. or higher, more preferably 74×10⁻⁷/° C. orhigher, and even more preferably 76×10⁻⁷/° C. or higher.

The glass transition point (Tg) is preferably 500° C. or higher, morepreferably 520° C. or higher, and even more preferably 540° C. orhigher, in order to reduce warpage after the chemical strengthening. Interms of easiness to be formed by a float forming process, the glasstransition point (Tg) is preferably 750° C. or lower, more preferably700° C. or lower, even more preferably 650° C. or lower, particularlypreferably 600° C. or lower, and most preferably 580° C. or lower.

A temperature (T2) at which the viscosity is 10² dPa·s is preferably1,750° C. or lower, more preferably 1,700° C. or lower, particularlypreferably 1,650° C. or lower, and typically 1,600° C. or lower. Thetemperature (T2) is a temperature indicating a melting temperature ofthe glass. As T2 is lower, the glass tends to be manufactured moreeasily. The lower limit of T2 is not particularly limited. However, aglass having a low T2 may be poor in stability. Therefore, T2 isnormally 1,400° C. or higher, and preferably 1,450° C. or higher.

In addition, a temperature (T4) at which the viscosity is 10⁴ dPa·s ispreferably 1,350° C. or lower, more preferably 1,250° C. or lower, evenmore preferably 1,200° C. or lower, and particularly preferably 1,150°C. or lower. The temperature (T4) is a temperature indicating atemperature at which the glass is formed into a sheet shape. A glasshaving a higher T4 tends to cause a higher load on a forming apparatus.The lower limit of T4 is not particularly limited. However, a glasshaving a low T4 may be poor in stability. Therefore, T4 is normally 900°C. or higher, preferably 950° C. or higher, and more preferably 1,000°C. or higher.

The devitrification temperature of the glass of the present invention ispreferably not higher than a temperature of 120° C. higher than thetemperature (T4) at which the viscosity is 10⁴ dPa·s, sincedevitrification does not occur easily when the glass is formed by afloat process. The devitrification temperature is more preferably nothigher than a temperature of 100° C. higher than T4, more preferably nothigher than a temperature of 50° C. higher than T4, and particularlypreferably not higher than T4.

The crystal growth rate at 850 to 1,200° C. in the glass of the presentinvention is preferably 600 μm/h or lower, since devitrification doesnot occur easily. The crystal growth rate at 850 to 1,200° C. is morepreferably 500 μm/h or lower, even more preferably 400 μm/h or lower,and particularly preferably 300 μm/h or lower. In addition, the maximumcrystal growth rate at 700 to 1,200° C. is preferably 600 μm/h or lower.

In addition, the crystal growth rate at 950° C. in the glass of thepresent invention is preferably 600 μm/h or lower, more preferably 500μm/h or lower, even more preferably 400 μm/h or lower, and particularlypreferably 300 μm/h or lower.

In addition, it is preferable that the glass of the present invention isnot devitrified when it is cooled on the aforementioned float simulationtemperature dropping conditions.

The softening point of the glass of the present invention is preferably850° C. or lower, more preferably 820° C. or lower, and even morepreferably 790° C. or lower. As the softening point of the glass islower, the heat treatment temperature for bending forming is lower sothat the energy consumption can be reduced, and the load on theequipment can be also reduced. In order to lower the bending formingtemperature, it is more preferable that the softening point is lower.However, a normal glass for chemical strengthening has a softening pointof 700° C. or higher. A glass having an excessively low softening pointcan easily relax a stress introduced in the chemical strengtheningtreatment so that the glass tends to have low strength. Therefore, thesoftening point is preferably 700° C. or higher. The softening point ismore preferably 720° C. or higher, and even more preferably 740° C. orhigher.

The softening point can be measured by a fiber elongation methodaccording to JIS R3103-1: 2001.

In the glass of the present invention, it is preferable that acrystallization peak temperature measured by the following measuringmethod is higher than the softening point. In addition, it is morepreferable that no crystallization peak is recognized.

That is, a glass of about 70 mg is crushed and ground in an agatemortar, and measured by a differential scanning calorimetry (DSC) from aroom temperature to 1,000° C. at a temperature rising rate of 10°C./min.

The surface compressive stress value (CS1) in a case where the glass ofthe present invention is chemically strengthened by immersion in sodiumnitrate at 450° C. for 1 hour is preferably 300 MPa or more, morepreferably 350 MPa or more, and even more preferably 400 MPa or more. Inorder to enhance the strength, it is more preferable that the CS1 islarger. However, in order to inhibit cracking during strengthening inthe chemical strengthening treatment step, the CS1 is, for example,preferably 800 MPa or less, more preferably 600 MPa or less, and evenmore preferably 500 MPa or less.

In addition, the depth of compressive stress layer (DOL1) in this caseis preferably 70 μm or more, more preferably 80 μm or more, even morepreferably 90 μm or more, and particularly preferably 100 μm or more. Onthe other hand, the upper limit of the DOL1 is not particularly limited.However, in a case of considering reduction in yield caused by crackingduring strengthening in the chemical strengthening treatment step, theDOL1 is, for example, preferably 200 μm or less, more preferably 150 μmor less, even more preferably 130 μm or less, and particularlypreferably 120 μm or less.

Incidentally, the CS1 and the DOL1 can be measured by a scattered-lightphotoelastic stress meter (such as SLP-1000 made by OriharaManufacturing Co., LTD.). Alternatively, they can be measured in thefollowing procedure using a birefringence imaging system Abrio-IM madeby Tokyo Instruments, Inc.

A section of a chemically strengthened glass having a size of 10 mm×10mm or more and a thickness of about 0.2 mm to 2 mm is polished andthinned into a range of 150 to 250 μm. A thinned sample obtained thuswhich has been thinned to a thickness of about 200 μm to 1 mm ismeasured by transmitted light using monochrome light having a wavelengthof 546 nm as a light source, so as to measure a phase difference(retardation) belonging to the chemically strengthened glass by thebirefringence imaging system. A stress is calculated from a valueobtained thus and the following Expression (2).

1.28×F=δ/(C×t′)  (2)

In Expression (2), F designates a stress [unit: MPa], δ designates aphase difference [unit: nm], C designates a photoelastic constant [unit:nm/cm/MPa], and t′ designates a sample thickness [unit: cm].

In the case where the glass of the present invention is chemicallystrengthened by immersion in sodium nitrate at 450° C. for 3 hours andsubsequent immersion in potassium nitrate at 450° C. for 1.5 hours, thesurface compressive stress value CS2 caused by an Na—K ion-exchangedlayer in this case is preferably 800 MPa or more, more preferably 850MPa or more, even more preferably 900 MPa or more, particularlypreferably 950 MPa or more, and further 1,000 MPa or more. On the otherhand, the upper limit of the CS2 is not particularly limited. In orderto make the reduction in yield caused by cracking during strengtheningin the chemical strengthening treatment step as small as possible, theCS2 is preferably 1,500 MPa or less, more preferably 1,300 MPa or less,even more preferably 1,200 MPa or less, and particularly preferably1,100 MPa or less.

The CS2 and the DOL2 can be, for example, measured by a surface stressmeter FSM-6000 made by Orihara Manufacturing Co., LTD.

Further, in the case where the glass of the present invention ischemically strengthened by immersion in sodium nitrate at 450° C. for 3hours and subsequent immersion in potassium nitrate at 450° C. for 1.5hours, the depth of compressive stress layer DOL3 caused by an Li—Naion-exchanged layer in this case is preferably 100 μm or more, morepreferably 110 μm or more, even more preferably 120 μm or more, andparticularly preferably 130 μm or more. On the other hand, the upperlimit of the DOL3 is not particularly limited. In a case of consideringthe reduction in yield caused by cracking during strengthening in thechemical strengthening treatment step, for example, the DOL3 ispreferably 200 μm or less, more preferably 180 μm or less, even morepreferably 170 μm or less, and particularly preferably 160 μm or less.

The DOL3 can be measured by a scattered-light photoelastic stress meter(such as SLP-1000 made by Orihara Manufacturing Co., LTD.), or can bemeasured in the aforementioned method using the birefringence imagingsystem Abrio-IM made by Tokyo Instruments, Inc.

In order to increase the surface compressive stress caused by thechemical strengthening, the fictive temperature of the glass of thepresent invention is preferably not higher than a temperature of 80° C.higher than the glass transition point (Tg) (hereinafter referred to as“Tg+80° C.”), more preferably Tg+50° C. or lower, even more preferablyTg+40° C. or lower, further more preferably Tg+30° C. or lower, evenfurther more preferably Tg+20° C. or lower, and particularly preferablyTg+10° C. or lower.

When the glass is obtained by a method in which glass raw materials aremelted at a high temperature and then cooled down, the fictivetemperature of the glass becomes lower as the cooling rate after themelting decreases. Therefore, in order to obtain a glass having a verylow fictive temperature, the glass has to be cooled slowly in a longtime. When the glass is cooled slowly, a devitrification phenomenon thatcrystals are precipitated during the cooling may occur easily in someglass composition. Therefore, in consideration of the manufacturingefficiency of the glass and inhibition of the devitrificationphenomenon, the fictive temperature is preferably Tg−30° C. or higher,more preferably Tg−10° C. or higher, and even more preferably Tg orhigher.

Incidentally, the fictive temperature of the glass can be obtainedexperimentally from the refractive index of the glass. A plurality ofglass pieces each having the same composition and having differentfictive temperatures are prepared in advance by a method in which eachglass retained at a certain temperature is quenched from thattemperature. The fictive temperatures of the glass pieces coincide withtemperatures at which the glass pieces had been retained before theywere quenched. Thus, the refractive indexes of the glass pieces aremeasured so that a calibration curve plotting the refractive indexeswith respect to the fictive temperatures can be created. FIG. 3 shows anexample of the calibration curve. Even for a glass with an unknowncooling rate or the like, the fictive temperature can be obtained fromthe calibration curve by measuring the refractive index of the glass.

However, another calibration curve has to be used for another glasscomposition. Therefore, it is necessary to use a calibration curvecreated by use of a glass having the same composition as a glass whosefictive temperature should be obtained.

The fictive temperature of a glass depends on a cooling rate at whichthe molten glass was cooled down. The fictive temperature tends toincrease as the cooling rate increases and tends to decrease as thecooling rate decreases. In addition, as the fictive temperature islower, the surface compressive stress after the chemical strengtheningtends to increase.

The glass for chemical strengthening of the present invention can bemanufactured by a common method. For example, raw materials ofcomponents of the glass are prepared, and then heated and melted in aglass melting furnace. After that, the glass is homogenized by a knownmethod, and formed into a desired shape such as a glass sheet, followedby annealing.

Examples of the method for forming the glass sheet include a floatprocess, a press process, a fusion process, and a down draw process.Particularly the float process suitable for mass production ispreferred. Alternatively, a continuously forming method other than thefloat process, for example, the fusion process and the down draw processare also preferred.

After that, the formed glass is ground and polished if necessary. Thus,a glass substrate is formed. Incidentally, when the glass substrate iscut into a predetermined shape with a predetermined size or the glasssubstrate is chamfered, it is preferable that cutting or chamfering isperformed before a chemical strengthening treatment which will bedescribed later, so that a compressive stress layer can be also formedin an end face by the chemical strengthening treatment.

<Chemically Strengthened Glass>

The chemically strengthened glass of the present invention has a basecomposition the same as the aforementioned glass composition of theglass for chemical strengthening. The surface compressive stress valueof the chemically strengthened glass of the present invention ispreferably 800 MPa or more.

The chemically strengthened glass of the present invention can bemanufactured by applying a chemical strengthening treatment to theobtained glass sheet, then washing and drying the glass sheet.

The chemical strengthening treatment can be performed by a known method.In the chemical strengthening treatment, the glass sheet is brought intocontact with a melt of metal salt (such as potassium nitrate) containingmetal ions (typically K ions) having large ionic radii by an immersionor the like, so that metal ions (typically Na ions or Li ions) havingsmall ionic radii in the glass sheet are replaced by the metal ions(typically K ions for the Na ions, and Na ions for the Li ions) havinglarge ionic radii.

The chemical strengthening treatment (ion exchange treatment) is notparticularly limited. For example, the chemical strengthening treatmentcan be performed by immersing the glass sheet into a molten salt ofpotassium nitrate or the like heated to 360 to 600° C. for 0.1 hours to500 hours. Incidentally, the heating temperature of the molten salt ispreferably 375 to 500° C., and the immersing time of the glass sheet inthe molten salt is preferably 0.3 hours to 200 hours.

Examples of the molten salt for the chemical strengthening treatmentinclude nitrates, sulfates, carbonates, and chlorides. Among them,examples of the nitrates include lithium nitrate, sodium nitrate,potassium nitrate, cesium nitrate, and silver nitrate. Examples of thesulfates include lithium sulfate, sodium sulfate, potassium sulfate,cesium sulfate, and silver sulfate. Examples of the carbonates includelithium carbonate, sodium carbonate, and potassium carbonate. Examplesof the chlorides include lithium chloride, sodium chloride, potassiumchloride, cesium chloride, and silver chloride. Each of those moltensalts may be used alone, or a plurality of kinds of them may be used incombination.

The treatment conditions of the chemical strengthening treatment are notparticularly limited in the present invention. Suitable conditions maybe selected in consideration of the characteristics and composition ofthe glass, the kind of molten salt, and the chemical strengtheningproperty such as the surface compressive stress and the depth ofcompressive stress layer desired in the chemically strengthened glassobtained finally, etc.

In the present invention, the chemical strengthening treatment may beperformed only once. Alternatively, a plurality of times of the chemicalstrengthening treatment (multistage strengthening) may be performed ontwo or more different conditions. Here, for example, as a first stage ofthe chemical strengthening treatment, a chemical strengthening treatmentis performed on conditions where the DOL can be increased and the CS canbe decreased relatively. After that, as a second stage of the chemicalstrengthening treatment, a chemical strengthening treatment is performedon conditions where the DOL can be decreased and the CS can be increasedrelatively. Thus, it is possible to suppress an internal tensile stressarea (St) while increasing the CS in the outermost surface of thechemically strengthened glass. As a result, it is possible to suppressthe internal tensile stress (CT) to a low value.

When the glass for chemical strengthening of the present invention has asheet-like shape (glass sheet), the sheet thickness (t) thereof is notparticularly limited. However, in order to enhance the chemicalstrengthening effect, the thickness is, for example, 2 mm or less,preferably 1.5 mm or less, more preferably 1 mm or less, even morepreferably 0.9 mm or less, particularly preferably 0.8 mm or less, andmost preferably 0.7 mm or less. On the other hand, in order to obtain aneffect of improving the strength sufficiently by the chemicalstrengthening treatment, the sheet thickness is, for example, 0.1 mm ormore, preferably 0.2 mm or more, more preferably 0.4 mm or more, andeven more preferably 0.5 mm or more.

The shape of the glass of the present invention may have another shapethan the sheet-like shape in accordance with a product, a usage, or thelike to which the glass should be applied. In addition, the glass sheetmay have a bordered shape different in peripheral thickness, or thelike. Further, the form of the glass sheet is not limited thereto. Forexample, the glass sheet may have two main surfaces not parallel to eachother, or one of the two main surfaces or all or a part of the both maybe curved. More specifically, the glass sheet may be, for example, aflat glass sheet free from warpages or a curved glass sheet having acurved surface.

The glass of the present invention is useful particularly as a coverglass for use in a mobile apparatus or the like such as a cellularphone, a smartphone, a personal digital assistant (PDA), or a tabletterminal. Further, it is also useful as a thing not intended to beportable, such as a cover glass for a display device such as atelevision (TV), a personal computer (PC), or a touch panel, an elevatorwall surface, a wall surface (whole-surface display) of an architecturesuch as a house or a building, a construction material such as a windowglass, a table top, or an interior decoration for a car, an airplane orthe like or a cover glass for them. In addition, the glass of thepresent invention is also useful for an application such as a housingwhich has not a sheet-like shape but a curved shape formed by bending orforming.

Examples

The present invention will be described along its examples below.However, the present invention is not limited thereto. Example 25 is acomparative example, and the other examples are working examples.Incidentally, as for respective measurement results in the tables,blanks designate unmeasured items.

(Preparation of Glass for Chemical Strengthening)

Glass sheets were prepared by melting in a platinum crucible so as tohave glass compositions represented by mole percentage on an oxide basisshown in Tables 3 to 6 respectively. Glass raw materials used generally,such as oxides, hydroxides, carbonates, or nitrates, were selectedsuitably, and weighed to obtain a glass of 1,000 g. Next, the mixedglass raw materials were put into the platinum crucible, and placed in aresistance-heating type electric furnace at 1,500 to 1,700° C. Thus, theglass raw materials were melted for about 3 hours to be degassed andhomogenized. The obtained molten glass was poured into a mold, andretained at a temperature of 50° C. higher than a glass transition pointfor 1 hour. After that, the molten glass was cooled down to a roomtemperature at a rate of 0.5° C./min. Thus, a glass block was obtained.The obtained glass block was cut and ground, and the both sides thereofwere finished into mirror surfaces, thereby obtaining a sheet-like glassmeasuring 50 mm in length, 50 mm in width and 0.8 mm in thickness.

Physical properties of each glass obtained thus was evaluated asfollows. Results are shown in Tables 3 to 6.

<Density>

Density was measured by an in-liquid weighing method (JIS Z8807: 2012Methods of measuring density and specific gravity of solid). The unit isg/cm³.

<Young's Modulus>

As for each glass that had not been chemically strengthened yet, theYoung's modulus (E) (unit: GPa) was measured by an ultrasonic pulsemethod (JIS R1602: 1995).

<Average Linear Expansion Coefficient and Glass Transition Point (Tg)>

The average linear expansion coefficient (α50-350) at a temperature of50 to 350° C. (unit: 10⁻⁷/° C.) and the glass transition point weremeasured according to a method of JIS R3102: 1995 “Testing method foraverage linear thermal expansion of glass”.

<T2, T4>

As for each glass that had not been chemically strengthened yet, thetemperature T2 at which the viscosity was 10² dPa·s and the temperatureT4 at which the viscosity was 10⁴ dPa·s were measured by a rotaryviscometer (according to ASTM C 965-96)

<DSC Peak Height>

DSC measurement was carried out by the aforementioned method to measurea peak height (unit: mcal/s).

<Crystal Growth Rate>

The crystal growth rate was measured in the following procedure.

A glass piece was crushed in a mortar, and classified. Grass particleswhich had been passed through a sieve of 3.35 mm meshes but had not beenpassed through a sieve of 2.36 meshes were washed with ion-exchangedwater, and dried. The glass particles obtained thus were used fortesting.

As shown in FIG. 4 , glass particles 3 were put one by one on concaveportions 2 of a long and narrow platinum cell (platinum cell 1 forevaluating devitrification) having a large number of concave portions,and heated in an electric furnace at 1,000 to 1,100° C. until thesurfaces of the glass particles were melted to be smooth.

Next, the glass was placed into a furnace with a temperature gradient,which was kept in a predetermined temperature, so that the glass wassubjected to a heat treatment for a fixed time (T). After the heattreatment, the glass was transferred to a room temperature so as to bequenched. Owing to this method, a large number of glass particles can beheated simultaneously by placing the long and narrow vessel in thefurnace with a temperature gradient. Thus, a maximum crystal growth ratecan be measured within a predetermined temperature range.

The glass subjected to the heat treatment was observed by a polarizingmicroscope (ECLIPSE LV100ND made by Nikon Corporation). Among observedcrystals, the diameter (L μm) of the largest one was measured.Observation was performed on conditions of an eyepiece of 10 power, anobjective lens of 5 to 100 power, transmitted light and polarizingobservation. A crystal caused by devitrification may be considered togrown up isotropically. Therefore, the crystal growth rate is L/(2T)[unit: μm/h].

Incidentally, a crystal that was not precipitated from the interfacewith the vessel was selected as a crystal to be measured, becausecrystal growth in a metal interface tends to indicate a differentbehavior from that of crystal growth that occurs inside the glass or ina glass-atmosphere interface.

<Devitrification Temperature>

Crushed glass particles were put into a platinum dish, and subjected toa heat treatment for 17 hours in an electric furnace controlled to afixed temperature. After the heat treatment, the glass was observed by apolarizing microscope, and a devitrification temperature was estimatedby a method for evaluating presence or absence of devitrification. Forexample, “1050-1078° C.” written in the table means that the glass wasdevitrified by a heat treatment at 1,050° C. but the glass was notdevitrified by a treatment at 1,078° C. In this case, thedevitrification temperature was 1,050° C. or higher and lower than1,078° C.

<Devitrification in Float Simulation Temperature Dropping>

A columnar glass sample measuring about ϕ20 mm by 15 mm was preparedfrom the glass block. The columnar glass sample was put into a ϕ40 mmcrucible made of a platinum-gold alloy, and subjected to a hyperthermictreatment in an electric furnace simulating temperature droppingconditions in a float furnace. After that, presence or absence ofdevitrification on the atmospheric surface side (fire-polished side) ofthe sample was evaluated by an optical microscope. “N” in the tablesdesignates no recognition of devitrification.

Incidentally, glass viscosity η (unit: dPa·s) during temperaturedropping was made to follow that during float forming. That is, thetemperature was decreased for 5 minutes from 1,300° C. to a temperatureat which log η reached about 4.4. Next, the temperature was decreasedfor 25 minutes to a temperature at which log reached about 5.5. Afterthat, the temperature was decreased for 4 minutes to a temperature atwhich log η reached about 9.9. The glass was then cooled down at acooling rate low enough to prevent the glass from cracking. Therefore, atemperature program during the temperature dropping depends on the glasscomposition. A temperature program for Glass A1 in Table 1 is shown inFIG. 5 by way of example.

<Refractive Index>

A refractive index on d-line (He light source, wavelength 587.6 nm) wasmeasured by a precision refractometer (KPR-2000 made by ShimadzuCorporation).

<Photoelastic Constant>

A photoelastic constant was measured using a sodium lamp as a lightsource according to a method of compression on a circular platedescribed in Journal of the Ceramic Association, Japan, Vol. 87, (1979)No. 1010, p. 519.

<Chemical Strengthening Property>

The surface compressive stresses CS1 and CS3 (unit: MPa) and the depthof compressive stress layers DOL1 and DOL3 (unit: μm) were measured by ameter SLP1000 made by Orihara Manufacturing Co., LTD. The surfacecompressive stress (CS2) (unit: MPa) and the depth of compressive stresslayer (DOL2) (unit: μm) were measured by a surface stress meter FSM-6000made by Orihara Manufacturing Co., LTD.

Incidentally, the CS1 and the DOL1 in the tables designate a surfacecompressive stress and a depth of compressive stress layer afterone-stage strengthening in which the obtained glass for chemicalstrengthening was chemically strengthened by immersion in sodium nitrateat 450° C. for 1 hour, respectively. The CS2 and the DOL2 designate asurface compressive stress and a depth of compressive stress layer in anNa—K ion-exchanged layer after two-stage strengthening in which theobtained glass for chemical strengthening was chemically strengthened byimmersion in sodium nitrate at 450° C. for 3 hours and subsequentimmersion in potassium nitrate at 450° C. for 1.5 hours, respectively.In addition, the CS3 and the DOL3 designate a surface compressive stressand a depth of compressive stress layer in an Li—Na ion-exchanged layerafter two-stage strengthening in which the obtained glass for chemicalstrengthening was chemically strengthened by immersion in sodium nitrateat 450° C. for 3 hours and subsequent immersion in potassium nitrate at450° C. for 1.5 hours, respectively.

TABLE 3 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 SiO₂ 66.9 62.9 62.9 61.962.9 66.9 Al₂O₃ 10.5 10.0 9.0 9.0 10.0 11.0 B₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0P₂O₅ 0.0 0.0 0.0 0.0 1.0 0.0 MgO 3.9 6.9 6.9 6.9 6.9 5.9 CaO 0.2 0.2 0.20.2 0.2 0.2 SrO 0.0 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 ZnO0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.1 0.0 0.1 0.1 0.1 0.1 ZrO₂ 1.0 1.0 1.01.0 1.0 1.0 Y₂O₃ 0.5 1.0 2.0 3.0 1.0 1.0 Li₂O 10.5 11.0 11.0 11.0 11.08.0 Na₂O 4.8 5.8 5.8 5.8 4.8 4.8 K₂O 1.6 1.1 1.1 1.1 1.1 1.1 Sum 100.099.9 100.0 100.0 100.0 100.0 Value M 1048 1360 1457 1680 1391 1256 ValueI 476 254 127 97 526 420 Value I2 2.0 0.4 −2.4 −4.0 1.5 3.6 Density(g/cm³) 2.47 2.54 2.60 2.66 2.53 2.51 E (GPa) 85 91 90 93 88 88 α50-35077 84 84 86 89 67 (10⁻⁷/° C.) Tg (° C.) 554 548 557 568 563 611 T2 (°C.) 1599 1460 1446 1418 1483 1613 T4 (° C.) 1143 1057 1037 1022 10691186 Devitrification 1118 or 1050-1078 1099-1129 1198-1225 1070-10961181-1205 temperature lower (° C.) DSC peak height 2.47 1.42 0.83 0.762.67 2.78 mcal/s Crystal growth 433 306 103 77 524 421 rateDevitrification N N during float simulation temperature droppingRefractive 1.537 1.545 1.554 index Photoelastic 26.8 26.3 25.9 constantCS1 341 347 334 348 379 314 DOL1 100 85 82 81 110 111 CS2 861 991 10781219 1021 1205 DOL2 10.4 6.7 5.6 4.4 7.9 8.0 CS3 248 259 263 280 276 257DOL3 152 131 121 111 130 149 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 SiO₂ 65.162.9 62.9 65.9 64.6 Al₂O₃ 10.5 11.0 10.5 11.5 10.8 B₂O₃ 0.0 0.0 0.0 0.00.0 P₂O₅ 0.0 0.0 0.0 0.0 0.0 MgO 6.4 5.9 5.9 3.9 5.4 CaO 0.0 0.2 0.2 0.20.2 SrO 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 ZnO 0.0 0.0 0.0 0.00.0 TiO₂ 0.1 0.0 0.1 0.1 0.1 ZrO₂ 1.0 1.0 1.0 1.0 1.0 Y₂O₃ 1.0 1.0 1.00.5 0.8 Li₂O 9.5 11.0 11.0 10.5 10.3 Na₂O 5.3 5.8 5.8 4.8 5.3 K₂O 1.11.1 1.6 1.6 1.4 Sum 100.0 99.9 100.0 100.0 100.0 Value M 1296 1391 13471174 1276 Value I 338 372 181 573 363 Value I2 2.1 1.4 −0.1 3.1 1.7Density (g/cm³) 2.52 2.53 2.53 2.48 2.51 E (GPa) 89 90 89 86 87 α50-35077 82 85 77 80 (10⁻⁷/° C.) Tg (° C.) 577 553 541 565 561 T2 (° C.) 15601506 1495 1604 1553 T4 (° C.) 1132 1083 1072 1152 1118 Devitrification1131-1155 1110-1135 1045 or 1126 or 1093 or temperature lower lowerlower (° C.) DSC peak height 2.10 2.07 1.21 3.00 2.11 mcal/s Crystalgrowth 378 407 183 489 328 rate Devitrification during float simulationtemperature dropping Refractive index Photoelastic constant CS1 330 366344 367 347 DOL1 98 93 86 103 96 CS2 1098 1006 954 915 989 DOL2 7.3 7.58.3 10.3 8.6 CS3 258 264 262 261 260 DOL3 140 136 131 151 140

TABLE 4 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 SiO₂ 63.8 64.9 64.366.9 65.7 65.4 Al₂O₃ 10.7 10.5 10.6 10.0 10.2 10.9 B₂O₃ 0.0 0.0 0.0 0.00.0 0.0 P₂O₅ 0.0 0.0 0.0 0.0 0.0 0.0 MgO 5.6 4.9 5.3 4.3 5.4 4.0 CaO 0.20.2 0.2 0.2 0.2 0.2 SrO 0.0 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.00.0 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.1 0.1 0.1 0.1 0.1 0.1 ZrO₂ 1.01.0 1.0 1.0 1.3 1.0 Y₂O₃ 0.9 0.7 0.8 0.7 0.5 0.5 Li₂O 10.7 10.7 10.710.1 10.2 10.5 Na₂O 5.5 5.3 5.4 5.5 5.2 5.5 K₂O 1.5 1.6 1.6 1.2 1.2 1.5Sum 100.0 100.0 100.0 100.0 Value M 1311 1197 1254 1010 1241 1221 ValueI 272 328 300 303 395 338 Value I2 0.8 0.9 0.8 1.1 1.9 1.4 Density(g/cm³) 2.52 2.50 2.51 2.49 2.50 2.49 E (GPa) 87 86 86 85 86 85 α50-350(10⁻⁷/° C.) 82 81 82 78 77 80 Tg (° C.) 552 548 550 555 557 554 T2 (°C.) 1505 1547 1536 1571 1566 1576 T4 (° C.) 1091 1107 1101 1128 11281131 Devitrification 1045 or 1082 or 1076 or 1107 or 1127-1151 1131 ortemperature lower lower lower lower lower (° C.) DSC peak height mcal/s1.66 1.84 1.75 1.84 1.91 1.79 Crystal growth rate 366 223 295 279 342287 Devitrification N N during float simulation temperature droppingRefractive index 1.533 1.526 1.529 1.528 Photoelastic constant CS1 346342 344 307 341 343 DOL1 91 93 92 102 98 101 CS2 972 908 940 900 969 913DOL2 8.5 9.4 8.9 9.7 8.6 10.2 CS3 261 255 258 229 255 252 DOL3 136 141138 152 145 148 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex. 22 SiO₂ 64.5 63.4 66.165.2 65.2 Al₂O₃ 10.9 11.2 11.2 11.3 11.3 B₂O₃ 0.0 0.0 0.0 0.0 0.0 P₂O₅0.0 0.0 0.0 0.0 0.0 MgO 5.0 6.0 3.1 4.1 4.4 CaO 0.2 0.2 0.2 0.2 0.2 SrO0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 ZnO 0.0 0.0 0.0 0.0 0.0 TiO₂0.1 0.1 0.1 0.1 0.1 ZrO₂ 1.0 1.2 1.3 1.4 1.4 Y₂O₃ 0.5 0.0 0.5 0.3 0.0Li₂O 10.7 10.8 10.4 10.3 10.3 Na₂O 5.5 5.6 5.6 5.6 5.6 K₂O 1.5 1.5 1.51.5 1.5 Sum Value M 1194 1301 1159 1261 1222 Value I 333 344 358 345 348Value I2 1.7 2.7 1.5 2.0 2.6 Density (g/cm³) 2.49 2.48 2.49 2.48 2.47 E(GPa) 86 85 85 85 84 α50-350 (10⁻⁷/° C.) 81 82 81 82 83 Tg (° C.) 546537 558 562 549 T2 (° C.) 1551 1536 1586 1586 1587 T4 (° C.) 1111 11051142 1141 1142 Devitrification 1112 or 1131 or 1120-1146 1190 or1167-1191 temperature lower higher higher (° C.) DSC peak height 1.941.78 1.96 1.82 1.80 mcal/s Crystal growth rate 334 289 320 280 278Devitrification N during float simulation temperature droppingRefractive index 1.529 1.528 1.526 1.527 1.524 Photoelastic constant CS1344 359 339 346 344 DOL1 95 91 107 103 102 CS2 902 903 895 931 901 DOL29.5 9.0 11.0 10.3 10.4 CS3 252 263 246 256 253 DOL3 144 141 154 149 150

TABLE 5 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28 Ex. 29 SiO₂ 64.4 64.870.0 69.9 66.9 67.5 71.4 Al₂O₃ 11.2 11.5 10.0 7.5 11.5 9.6 9.0 B₂O₃ 0.00.4 0.0 0.0 0.0 0.0 0.0 P₂O₅ 0.0 0.0 0.0 0.0 0.0 1.0 0.0 MgO 5.0 0.0 5.07.0 2.9 5.4 3.5 CaO 0.2 0.8 0.0 0.2 0.2 0.2 0.2 SrO 0.0 0.1 0.0 0.0 0.00.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZnO 0.0 0.1 0.0 0.0 0.0 0.0 0.0TiO₂ 0.1 0.0 0.0 0.1 0.1 0.1 0.1 ZrO₂ 1.4 1.8 1.0 1.0 1.0 1.0 1.0 Y₂O₃0.0 0.0 0.0 0.0 0.0 0.0 1.0 Li₂O 10.5 10.6 10.0 8.0 10.5 10.1 9.0 Na₂O5.6 10.0 3.0 5.3 4.8 4.0 4.0 K₂O 1.6 0.0 1.0 1.0 2.1 1.0 0.8 Sum 100.0100.0 100.0 100.0 100.0 100.0 Value M 1281 866 973 679 986 946 845 ValueI 312 −343 1052 −22 466 723 659 Value I2 2.3 −1.7 6.2 0.9 2.8 4.1 2.5Density (g/cm³) 2.47 2.47 2.42 2.44 2.45 2.44 2.48 E (GPa) 85 84 84 8385 85 85 α50-350 81 89 63 72 78 73 66 (10⁻⁷/° C.) Tg (° C.) 550 513 585548 550 567 593 T2 (° C.) 1564 1551 1675 1629 1632 1625 1675 T4 (° C.)1126 1088 1211 1159 1165 1173 1203 Devitrification 1175 or 1194-12001090-1100 1073-1100 1144-1170 temperature higher (° C.) DSC peak height1.58 0.55 4.62 0.45 2.57 3.55 3.31 mcal/s Crystal growth 228 20 1078 24477 696 691 rate Devitrification N during float simulation temperaturedropping Refractive index 1.526 1.517 1.517 Photoelastic constant 29.128.9 CS1 350 229 380 203 345 345 303 DOL1 96 129 108 97 103 124 117 CS2903 742 906 908 781 875 981 DOL2 10.0 12.2 9.0 8.8 12.5 10.2 9.3 CS3 260151 261 193 248 244 226 DOL3 145 173 164 155 157 155 166

TABLE 6 Ex. 30 Ex. 31 Ex. 32 Ex. 33 Ex. 34 Ex. 35 Ex. 36 Ex. 37 SiO₂67.0 66.5 66.0 68.5 66.5 66.5 64.0 64.0 Al₂O₃ 10.8 10.3 12.0 10.4 11.710.6 13.0 13.0 B₂O₃ P₂O₅ MgO 3.7 3.2 3.9 4.4 3.3 5.4 3.6 3.6 CaO 0.2 0.20.2 0.2 0.2 0.2 0.2 0.2 SrO BaO ZnO TiO₂ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1ZrO₂ 1.3 1.3 0.8 1.1 1.3 1.3 1.3 1.0 Y₂O₃ 0.5 0.5 0.5 0.3 0.5 0.5 0.50.8 Li₂O 10.5 11.6 10.0 10.1 10.5 10.0 11.2 10.5 Na₂O 4.5 5.0 5.3 3.94.5 4.1 4.7 5.4 K₂O 1.5 1.5 1.3 1.2 1.5 1.4 1.5 1.5 Sum 100.0 100.0100.0 100.0 100.0 100.0 100.0 Value M 1281 1141 1098 1062 1282 1320 15071382 Value I 312 537 532 818 715 661 833 579 Value I2 2.3 2.8 2.5 2.52.5 2.5 2.5 2.5 Density (g/cm³) 2.49 2.48 2.48 2.46 2.48 2.49 2.50 2.51E (GPa) 88 85 85 85 86 86 87 87 α50-350 75 80 77 70 75 73 78 80 (10⁻⁷/°C.) Tg (° C.) 566 539 573 571 577 576 577 582 T2 (° C.) 1603 1569 16191638 1623 1601 1592 1595 T4 (° C.) 1156 1115 1164 1179 1170 1160 11521152 Devitrification 1150 -1171 1171-1200 1220-1250 1170-1200temperature (° C.) DSC peak height 2.84 2.29 3.21 3.75 3.37 3.00 3.893.31 mcal/s Crystal growth 570 455 600 822 676 613 782 616 rateDevitrification during float simulation temperature dropping Refractive1.525 1.526 1.525 1.522 1.526 1.527 1.530 1.530 index Photoelastic 28.227.9 27.9 28.5 28.1 28.0 27.5 27.4 constant CS1 369 368 348 368 390 375439 391 DOL1 104 99 110 107 109 101 107 109 CS2 923 803 945 920 956 1023992 1015 DOL2 10.1 10.4 10.2 9.4 10.3 8.5 9.7 10.0 CS3 265 254 246 258273 278 296 273 DOL3 152 151 157 158 154 145 148 149

In each of the glasses in Examples 1 to 23, 30 to 32, and 37 having thevalue M of 1,000 or more, the value I of 600 or less and the value I2 of5 or less, it is understood that the surface compressive stress CS1obtained by the one-stage strengthening is large, the surfacecompressive stress CS2 obtained by the two-stage strengthening is large,and the crystal growth rate is low. This means that for the glasscompositions of those glasses, defects of devitrification hardly occurand high yield can be expected in a mass-production process such as afloat process, and high strength can be exhibited in practical use as acover glass or the like.

In each of Examples 33 to 36 having the value M of 1,000 or more and thevalue I2 of 5 or less while having the value I being slightly large, theglass tends to be devitrified a little easily due to its devitrificationtemperature being higher than in Example 1, etc. However it isunderstood that the glass can obtain an excellent strength by thechemical strengthening.

In each of Examples 24, 26, and 27 having the values I and 12 beingsmall while having the value M being smaller than 1,000, it isunderstood that the surface compressive stress caused by the chemicalstrengthening may be insufficient, but the crystal growth rate is lowand the glass is not devitrified easily.

In Example 25 having the value M being small and the values I and 12being large, the glass has a crystal growth rate too high to bemanufactured easily.

Although the present invention has been described in detail withreference to its specific embodiments, it is obvious for those in theart that various changes or modifications can be made without departingfrom the spirit and scope of the present invention. The presentapplication is based on a Japanese patent application (Japanese PatentApplication No. 2018-018508) filed on Feb. 5, 2018, the contents ofwhich are incorporated by reference.

REFERENCE SIGNS LIST

-   1 platinum cell for evaluating devitrification-   2 concave portion-   3 glass particle

1. A glass for chemical strengthening comprising, in mole percentage onan oxide basis: 60 to 72% of SiO₂; 9 to 20% of Al₂O₃; 1 to 15% of Li₂O;0.1 to 5% of Y₂O₃; 0 to 1.5% of ZrO₂; and 0 to 1% of TiO₂, having atotal content of one or more kinds of MgO, CaO, SrO, BaO and ZnO of 1 to10%, having a total content of Na₂O and K₂O of 1.5 to 10%, having atotal content of B₂O₃ and P₂O₅ of 0 to 10%, wherein a ratio([Al₂O₃]+[Li₂O])/([Na₂O]+[K₂O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]+[ZrO₂]+[Y₂O₃])is from 0.7 to 3, wherein a ratio [MgO])/([CaO]+[SrO]+[BaO]+[ZnO]) isfrom 10 to 45, and having a value M expressed by the followingexpression of 1,100 or more:M=−5×[SiO₂]+1×[Al₂O₃]+50×[Li₂O]−35×[Na₂O]+32×[K₂O]+85×[MgO]+54×[Ca0]−41×[SrO]−4×[P₂O₅]+218×[Y₂O₃]+436×[ZrO₂]−1180, wherein each of [SiO₂],[Al₂O₃], [Li₂O], [Na₂O], [K₂O], [MgO], [CaO], [SrO], [P₂O₅], [Y₂O₃], and[ZrO₂] designates a content of each component in mole percentage on anoxide basis.
 2. The glass for chemical strengthening according to claim1, comprising, in mole percentage on an oxide basis: 1 to 10% of Na₂O.3. The glass for chemical strengthening according to claim 1,comprising, in mole percentage on an oxide basis: 0.5 to 10% of K₂O. 4.The glass for chemical strengthening according to claim 1, comprising,in mole percentage on an oxide basis: 1 to 10% of MgO.
 5. The glass forchemical strengthening according to claim 1, having the total content ofNa₂O and K₂O of 1.5 to 8% in mole percentage on an oxide basis.
 6. Theglass for chemical strengthening according to claim 1, having the totalcontent of B₂O₃ and P₂O₅ of 0 to 6% in mole percentage on an oxidebasis.
 7. The glass for chemical strengthening according to claim 1,having a temperature (T4) at which a viscosity is 10⁴ dPa·s of 1,350° C.or lower.
 8. The glass for chemical strengthening according to claim 1,wherein when the glass is chemically strengthened by an immersion in asodium nitrate at 450° C. for 1 hour, a surface compressive stress valuethereof is 300 MPa or more, and when the glass is chemicallystrengthened by an immersion in a sodium nitrate at 450° C. for 3 hourand a subsequent immersion in a potassium nitrate at 450° C. for 1.5hours, a surface compressive stress value thereof is 800 MPa or more.